Cold-worked stainless steel



United States Patent 3,340,048 COLD-WORKED STAINLESS STEEL StephenFloreen, Westfield, NJ., assignor to The International Nickel Company,Inc., New York, N.Y., a

corporation of Delaware No Drawing. Filed Mar. 31, 1964, Ser. No.356,037

5 Claims. (Cl. 75-128) The present invention relates to production ofstainless steel products and more particularly to production ofstainless steelv products characterized by improved high strengthandtoughness qualities.

Heretofore, the art has endeavored to produce stainless steel products,for instance, stainless steel sheet and strip, having to a high degreespecial characteristics which are required of metal products for use inhighly stressed corrosion resistant structures. It is well known thatsome austenitic stainless steels are improved in strength by coldworking and stainless steels of well known compositions have beenproduced for many years as cold rolled products. Although cold-workedstainless steels have been highly satisfactory for many uses, there areneeds in industry and commerce for stainless steels having improvedcharacteristics. A particular problem in the improvement of stainlesssteels involves providing stainless steel products which arecharacterized not only by improved high strength such as yield strengthbut in addition by adequate sharp-notch toughness. Sharp-notch toughnessis the capability or capacity of a metal to yield plastically under highlocalized stress such as might occur at the root of a sharp notch andsteels having adequate sharpnotch toughness are referred to as beingsharp-notch ductile. The sharp-notch (tensile strength) tensile strengthratio is used as a criterion of toughness and, as referred to herein forsheet specimens, metals characterized by sharpnotch/tensile strengthratios of at least about 1, i.e., 0.95 or greater, are deemedsharp-notch ductile, whereas metals characterized by lower sharp-notch/tensile strength ratios are deemed notch brittle or notch sensitive.

In providing stainless steel products for commercial use in highlystressed structures, it is not only important to produce steels ofimproved strength and toughness qualities but, in addition, it is alsoimportant that such improved products can be produced by commerciallypractical production methods that do not involve unusually high cost.For instance, it is commercially advantageous that improved metalproducts can be produced by unidirectionally rolling, or other simpleworking techniques, the metal at temperatures which are typicallyemployed for cold working commercial stainless steels, without necessityfor resorting to cross-rolling or to unusually high or low workingtemperatures. Most conveniently and economically, working isaccomplished at room temperature.

A further aspect of the problem of producing high strength stainlesssteel products is that although it is possible to greatly strengthensome known stainless steels by cold rolling the steels to very highreductions in cross-sectional area, it has been found that stainlesssteels which are in the condition obtained by cold rolling tocross-sectional reductions of 50% or greater are often adverselycharacterized by poor characteristics in the direction transverse to therolling direction. Accordingly, there are needs for special stainlesssteel alloys and production processes therefor which enable productionof high strength, sharp-notch ductile stainless steel products withoutnecessity of cold working the steel to amounts of deformation equivalentto cross-sectional reductions in area of 50% or greater. Although manyattempts were made to overcome the foregoing difficulties and otherdisadvantages, none, as far as I am aware, was entirely successful whencarried into practice commercially on an industrial scale.

It has now been discovered that stainless steel produtcs characterizedby an improved combination of high strength and toughness can beproduced by a new commercially practical process involving working andheat treating a workpiece of a new stainless steel alloy.

It is an object of the present invention to provide a new process formaking high strength, sharp-notch ductile stainless steel products.

Another object of the invention is to provide improved stainless steelproducts, including stainless steel sheet, strip, bar, rod, wire, tubingand the like, produced by a special process.

The invention also contemplates providing a new stainless steel alloyhaving a controlled chemical composition that is of sufiicient scope toenable commercial production of stainless steel products which areconsistently characterized by a combination of high yield strength andsharp-notch ductility.

Other objects and advantages will become apparent from the followingdescription.

Generally speaking, the present invention contemplates production of ahigh strength, sharp-notch ductile stainless steel product in accordancewith a new strengthening process comprising the steps of providing anaustenitically structured workpiece of a special nickel-chromium-cobaltstainless steel alloy which is initially of a structure including atleast about austenite, plastically deforming the initiallyaustenitically structured workpiece at a temperature within the range ofabout 5 F. to about F. by an amount at least equivalent to about 20%reduction in cross-sectional area to thereby transform theaustenitically structured workpiece to a martensitically structuredworkpiece having at least about 60% to about 99% matensite and with theremainder of the structure substantially all austenite and thereafterheat treating the martensitically structured workpiece for about 1 hourto about 48 hours at about 750 F. to about 850 F.

The invention also provides a special nickel-chromiumcobalt stainlesssteel alloy, which is the special alloy of the workpiece required in theprocess of the invention, containing about 3% to about 8% nickel, about12% to about 17% chromium, about 7% to about 13% cobalt, about 0.01% toabout 1% manganese, about 0.01% to about 0.15% carbon, about 0.005% toabout 0.1% nitrogen, up to about 5% molybdenum, not more than about 0.15silicon, with the balance essentially iron and characterized by anequivalent nickel index (ENI) of at least about 19.5 but not greaterthan about 22 as computed by the formula ENI=percent Ni+0.68 (percentCr)+0.55 (percent Mn) +0.45 (percent Si)+27 (percent C+percent N) +0.2(percent Co) +percent Mo All alloy percentages set forth herein are byweight and in the foregoing formula, which is the equivalent nickelindex formula, the expressions percent Ni, percent Cr, percent Mn,percent Si, percent C, percent N, percent Co and percent Mo are theweight percentages of nickel, chromium, manganese, silicon, carbon,nitrogen, cobalt' and molybdenum, respectively, in the alloy of theinvention. Although the balance of the nickel-chromium-cobalt stainlesssteel alloy composition of the invention is characterized herein asbeing essentially iron, it is to be understood that the term balanceessentially iron does not exclude small amounts of other elements whichcan serve some useful purpose ancillary to the objects of the invention,e.g., up to about 0.2% aluminum, up to about 0.1% each of calcium,magnesium and/or zirconium, and up to about 0.01% boron. Aluminum,calcium, magnesium, zirconium, and/or boron can serve purposes ofdeoxidation, malleabilization and/or purification. Although smallamounts of silicon, e.g., 0.05% silicon, can be present, the maximumsilicon content of the alloy of the invention is restricted to about0.15% in order that the alloy be characterized by sharp-notch ductility.The balance of the alloy of the invention may contain small amounts ofrelatively harmless impurities such as up to 2% copper and may alsocontain very small amounts of other more detrimental impurities such assulfur, phosphorus, bismuth, antimony, tin, lead, arsenic, beryllium,etc. However, the total amount of these impurities must be less than0.03% of the steel, e.g., about 0.02% or less.

The alloy of the invention is characterized by an improved combinationof high strength, hardness and toughness, including a yield strengthwhich is consistently at least 240,000 pounds per square inch (p.s.i.)together with sharp-notch ductility, i.e., a sharp-notch/tensilestrength ratio of at'least about 1 or greater, when in a martensiticcondition obtained by plastically deforming and heat treating the alloyin accordance with the process of the invention. Thus, the presentinvention provides a new alloy characterized by ranges of chemicalcomposition and equivalent nickel index which are specially controlledto provide for consistently obtaining a high yield strength of at least240,000 p.s.i in combination with sharp-notch ductility and which arealso of sufficient scope to permit consistent commercial production ofspecific embodiments of this new high strength alloy on a practicalbasis and with the normal tolerances for variation in composition(within the ranges set forth herein) demanded for successful commercialpractice.

In carrying the invention into practice, the ingredients for thestainless steel are melted in an induction furnace or any of the otherfurnaces employed for production of similar alloys. Vacuum melting orinert atmosphere melting can be employed if desired but such practicesare not necessarily required for successfully practicing the invention.When the stainless steel products of the invention are producedcommercially, the workpieces will usually be hot Worked and sometimescold worked before being processed in accordance with the process of theinvention. Advantageously, workpieces which have been hot worked andespecially workpieces which have been cold worked are annealed prior tobeing subjected to a deformation step of the process of the invention. Asatisfactory annealing treatment is accomplished by heating theworkpiece for about 1 hour to about 24 hours at a temperature of about1800 F. to about 2100 F., e.g., about 1950 F., and thereafter aircooling to room temperature. Variations in annealing temperatures withinthe range 1800 F. to 2 l F. have little effect but annealing below 1800F. is to be avoided because of possible embrittlement by carbides orsigma phase.

The plastic deformation of the workpiece which is accomplished in theprocess of the invention can be performed by rolling the workpiece atnear room temperature and deformation temperatures in the range 5 F. to100 F. are satisfactory for obtaining good results. Rolling is normallyaccomplished by multiple passes and it is to be recognized that it maybe necessary to allow the workpiece to cool between passes in order thatthe workpiece temperature be within the range of 5 F. to 100 F. at thestart of each pass. The amount of reduction in cross-sectional area isadvantageously at least 20% and less than 50%, e.g., 45%. Reductions ofless than 20% do not provide for developing full strength in theproducts and larger reductions, e.g., 60%, can cause mechanicalanisotropy and poor transverse toughness in the product. Deformation inaccordance with the invention can be accomplished not only by rollingbut also by drawing, forging, spinning, or other forming techniques. Ofcourse,

it is to be understood that the deformation step can com;

prise a number of substeps, such as multiple passes through a rollingmill or through a series of drawing dies. It is to be emphasized thatthe workpiece is initially, i.e., at the start of deformation,austenitic in structure and that the working thereof in accordance withthe invention results in transformation of austenite to martensite. Todevelop the full strength of the alloy, plastic deformation inaccordance with the process of the invention accomplishes in successionplastic deformation of austenite, transformation of austenite tomartensite and plastic deformation of martensite.

The strengthening heat treatment which is accomplished in the process ofthe invention is performed by heating the workpiece, after deformationis complete, for a pe-.

riod of about 1 hour to about 48 hours at a temperature from about 750F. to 850 F. Either lower or higher heat treating temperatures will notdevelop the full strength of the products. Also, the full strength ofthe new stainless steel product will not be achieved if a heat treatmentof tooshort duration or a heat treatment of extremely long durationgreater than 48 hours is .performed instead of the strengthening heattreatment of the process of the invention.

It is advantageous in order to produce stainless steel products ofespecially high strength and hardness in combination with sharp-notchductility that the alloy of the invention contain about 4.5% to about5.5% nickel, about 14.5% to about 16% chromium, about 7.5% to about 10%cobalt, about 0.1% to about 0.5% manganese, about 0.04% to about 0.09%carbon, about 0.01% to about 0.05% nitrogen, not more than about 0.1%silicon, with the balance essentially iron and have an equivalent nickelindex of about 20 to about 21. Stainless steel products of thisadvantageous composition, when in a martensitically strucured conditionobtained by deforming a workpiece of this advantageous alloy by anamount equivalent to at least about 30% reduction in area and by heattreating at about 750 F. to about 850 F. for about 1 hour to about 48hours, advantageously about 4 hours to about 48 hours, are characterizedby especially high yield strengths of at least about 260,000 p.s.i. anda shar-p-notch/tensile strength ratio of about 1 or greater. Forinstance, Alloys 2 and 3 referred to hereinafter are particularlyadvantageous alloys of the invention.

For the purpose of giving those skilled in the art a betterunderstanding of the invention and a better appreciation of theadvantages of the invention, the production of stainless steel productsof alloys and by processes. in accordance with the invention as comparedto the production of other alloy products or by other processes isillustrated by the following examples. Nine heats of stainless steelalloys were melted in induction furnaces in air atmospheres and castinto ingot. Five of these heats (Alloys 1 tlirough S) are examples ofalloys within the invention and the chemical compositions of these fivealloys are set forth in Table I:

TABLE 1 Alloy No Percent Percent Percent Percent Percent Percent PercentPercent Percent. ENI.

Ni r C0 Mn Si O N Mo Fe 6.7 16.1 7.6 0.40 0.01 0.06 0.03 39.1.... 21.85.1 15.7 8.4 0.14 0.02 0.06 0.03 BaL-.. 20.1 5.1 14.6 7.7 0.24 0.05 0.05 0.03 2.05 Bal 20.6 5.2 12.7 7.8 0.07 0.05 0.07 0.03 3.90 Ba1 22.0 6.114.3 11.1 0.23 0.05 0.04 0.03 Bal 20.0

BaL=Balance including small am EN I=Equivalent Nickel Index.

5 The other four of these heats (Alloys W, X, Y and Z) are ofcompositions outside the scope of the invention. The compositions ofthese four Alloys W, X, Y and Z are set forth in Table II:

radii not greater than 0.001 inch. These tensile specimens, both thesmooth specimens and the sharp-notched specimens, were tested at roomtemperature at a strain rate of 0.005 inch per inch per minute until theyield strength TABLE II Alloy No Percent Percent Percent Percent PercentPercent Percent Percent Percent ENI Ni Cr Co Mn Si N Mo FeENI=Equivalent Nickel Index.

From Table II it is apparent that Alloys W and X contain only about 3.5%and about 3.1% cobalt, respectively, which amounts are less than theamount of cobalt required in the alloys of the invention. These lowcobalt contents of about 3.1% and about 3.5% are insufficient to providefor obtaining the high yield strength of at least about 240,000 p.s.i.which is a characteristic of the alloy of the invention. In order toprovide for consistently obtaining high strength and hardness, the alloyof the invention contains at least about 7% cobalt. Alloys Y and Z arecharacterized by equivalent nickel indexes of only about 18.7 and 18.5,respectively, whereas for the purpose of obtaining high strength andhardness, the alloy of the invention is characterized by an equivalentnickel index of about 19.5 to about 22.

Ingots of Alloys 1 through 5 and W through Z were hot forged and hotrolled to about one-quarter inch thick plates and then cold rolled toabout 0.1 inch thick sheet. These sheets were annealed by heat treatingfor one hour at 1950 F. and air coiling to room temperature to insurethat the sheets consisted predominantly of austenite and to eliminatehardening effects resulting from the prior hot and cold working steps.Thus, sheets of Alloys 1 through 5, which were characterized by astructure comprising at least about 90% austeuite, up to about 10%martensite and possibly also delta ferrite in small amounts not greaterthan about 1%, were provided as workpieces for carrying out the processof the invention.

The annealed sheets of Alloys 1 through 5 and Alloys W through Z werecold rolled at room temperature (about 70 F.) to a reduction of about40% in thickness, which reduction is approximately equal to across-sectional reduction in area of about 40%. Rolling was accomplishedin several passes and the sheets were cooled to room temperature betweenpasses. As a result of rolling, austenite in the sheets was transformedto martensite. After rolling was completed, the sheets, which were thenabout 0.06 inch thick, were heat treated for 24 hours at 800 F. for thepurpose of increasing the yield strength and insuring the toughnessthereof. Thus, finished stainless steel products of Alloys 1 through 5were produced in accordance with the invention along with finishedproducts of Alloys W through Z which were made by the same rolling andheat treating steps which were performed on Alloys 1 through 5.

To test characteristics and results of the foregoing alloys andprocesses, both smooth (unnotched) and sharp-notched sheet tensilespecimens were machined from the sheets. Longitudinal axes of thespecimens were in alignment with the direction in which the sheets wererolled. The dimensions of these sheet tensile specimens of one inchwidth were in accordance with dimensions for specimens shown in FIGURE 1in the paper of G. B. Espey, M. H. Jones and W. F. Brown, Jr., The SharpEdge Notch Tensile Strength of Several High-Strength Steel Sheet Alloys,published in American Society for Testing Materials Proceedings, volume59, 1959. Thus, the sharp-notched specimens used to test the stainlesssteels of the foregoing examples had stress-concentration factors, K ofat least 18 and sharp notches with root was reached and thereafter at anincreased strain rate of 0.05 inch per inch per minute until thespecimens fractured. The term yield strength, as employed herein, is theyield strength of 0.2% offset. The results of these tests are set forthin Tables HI and IV wherein the results in Table III showcharacteristics of examples in accordance with the invention and theresults in Table IV show characteristics of stainless steel products ofalloys which are not in accordance with the invention:

TABLE III 0.2% Yield Ultimate Sharp-Notch Alloy No. Strength, TensileTensile Ratio p.s.i. Strength, Strength, SNTS/TS p.s.i. p.s.i.

SNTS/TS Ratio of Sharp-Notch Tensile Strength to Ultimate TensileStrength.

TABLE IV 0.2% Yield Ultimate Sharp-Notch Alloy No. Strength, TensileTensile Ratio p.s.i. Strength, Strength, SN'IS/TS p.s.i. p.s.i.

TABLE V Heat Treating Temperature, 0.2% Yield Ultimate Tensile F.Strength (p.s.i.) Strength (p.s.i.)

N.H.T.=Not Heat Treated.

The results set forth in Tables III and IV, when taken in conjunctionwith the compositions set forth in Tables I and H, demonstrate thatAlloys 1 through 5 of the invention are all characterized by high yieldstrengths which are consistently greater than about 240,000 p.s.i. andby sharp-notch ductility whereas Alloys W, X, Y and Z which are notalloys in accordance with the invention do not possess the high yieldstrengths which characterize products made of alloys of the invention.In addition, results set forth in Table V illustrate the achievement ofsuperior strength that is obtained by heat treating in accordance withthe invention.

The present invention is particularly applicable to the production ofsharp-notch ductile stainless steel products characterized by highcorrosion resistance, hardness, yield strength and tensile strength.Stainless steel products that can be produced in accordance with theinvention include sheets, plates, strips, rods, bars, tubing, forgings,wire, extrusions, stampings and pressings. The alloy and process of theinvention are useful for making highly stressed structures and articlesfor use in corrosive environments and also in cryogenic temperatureenvironments. Such structures and articles include pipes, couplings,pressure vessels, barrels, wheel spokes, bolts, rivets and screws.Furthermore, the alloy and process of the invention are also useful formaking hard, corrosion resisting articles including knives, surgicalinstruments, dental tools, saws and chisels.

In addition, it is to be observed that the alloy of the invention is atransformable stainless steel that is particularly adapted for coldworking at near room temperature in the austenitic condition totransform a ustenite of the alloy into martensite and increase thestrength of the alloy and thereby produce a martensitically structuredalloy which can be heat treated to still further increase the strengththereof. Accordingly, it is to be understood that the alloy of theinvention is especially useful where it is desired to make stainlesssteel products and articles characterized by high strength incombination with sharpnotch ductility by methods involving plasticallydeforming a stainless steel alloy at or near room temperature.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the inven tion and appended claims.

I claim: 7

1. A wrought and heat treated martensitic-structured nickel-chromiumcobalt stainless steel consisting essentially of 3% to about 8% nickel,about 12% to about 17% chromium, about 7% to about 13% cobalt, about0.01% to about 1% manganese, about 0.01% to about 0.15% carbon, about0.005% to about 0.1% nitrogen, up to molybdenum, not more than about0.15% silicon, with the percentages of the aforesaid elementsproportioned to characterize the composition with an equivalent nickelindex (ENI) of at least 19.5 but not greater than 22 as computed by theformula ENI=percent Ni+0.68 (percent Cr) +0.55 (percent Mn)+0.45(percent Si) +27 (percent C +percent N)+0.2 (percent Co) +percent Mo andwith the balance essentially iron, cold worked to at least about 20% andless than 50% reduction in cross sectional area and thereafter heattreated for about 1 hour to about 48 hours at 750 F. to 850 F., having amicrostructure of at least about 60% to about 99% martensite andtheremainder essentially austeni-te and possessing a yield strength ofat least 240,000 pounds per square inch and a sharp-notch/tensilestrength ratio of at leastabout 1.

2. A stainless steel as set forth in claim 1 consisting ENI=percentNi+0.68 (percent Cr) +0.55 (percent Mn) +O.45 (percent Si) +27 (percentC +percent N)+0.2 (percent Co) +percent Mo and with the balanceessentially iron, cold worked to at least about 30% and less than 50%reduction in cross sectional area and thereafter heat treated for about4 hours to' about 48 hours at 750 F. to 850 F., having a microstructureof about 60% to 99% martensite .and the remainder essentially austeniteand possessing a yield strength of at least 260,000 pounds per squareinch and a sharp-notch/ tensile strength ratio of at least about 1.

3. An alloy consisting essentially of 3% to about 8% nickel, about 12%to about 17% chromium, about 7% to about 13% cobalt, about 0.01% toabout 1% manganese, about 0.01% to about 0.15% carbon, about 0.005% toabout 0.1% nitrogen, not more than about 0.15 silicon with the' balanceessentially iron and characterized by an equivalent nickel index (ENI)of at least about 19.5 but not greater than about 22 as computed by theformula ENI=percent Ni+0.68 (percent Cr) +0.55 (percent Mn)+0.45(percent Si) +27 (percent C +percent N) +0.2 (percent Co) ENI=percentNi+0.68 (percent Cr) +0.55 (percent Mn)'+0.45 (percent Si) +27 (percentC +percent N)+0.2 (percent Co) +percent Mo 5. An alloy as set forth inclaim 4 wherein the alloy consists essentially of nickel, chromium,cobalt, manganese, carbon, nitrogen and silicon in amounts in accordanceWith claim 4 and a balance that is iron.

References Cited UNITED STATES PATENTS 2,795,519 6/ 1957 Angel 148--122,990,275 6/ 1961 Binder -128 X 3,154,412 10/1964 Kasak 7512 8 X3,251,683 5/1966 Hammond 75128 DAVID L. RECK, Primary Examiner. P.WEINSTEIN, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,340,048 September 5, 1967 Stephen Floreen It is hereby certified thaterror appears in the above numbered patent requiring correction and thatthe said Letters Patent should read as corrected below.

Column 4, line 42, for "strucured" read structured column 7, line 48,before "5%" insert about Signed and sealed this 26th day of November1968.

(SEAL) Attest:

EDWARD J. BRENNER Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer

1. A WROUGHT AND HEAT TREATED MARTENSITIC-STRUCTUREDNICKEL-CHROMIUM-COBALT STAINLESS STEEL CONSISTING ESSENTIALLY OF 3% TOABOUT 8% NICKEL, ABOUT 12% TO ABOUT 17% CHROMIUM, ABOUT 7% TO ABOUT 13%COBALT, ABOUT 0.01% TO ABOUT 1% MANGANESE, ABOUT 0.01% TO ABOUT 0.15%CARBON, ABOUT 0.005% TO ABOUT 0.1% NITROGEN, UP TO 5% MOLYBDENUM, NOTMORE THAN ABOUT 0.15% SILICON, WITH THE PERCENTAGE OF THE AFORESAIDELEMENTS PROPORTIONED TO CHARACTERIZE THE COMPOSITION WITH AN EQUIVALENTNICKEL INDEX (ENI) OF AT LEAST 19.5 BUT NOT GREATER THAN 22 AS COMPUTEDBY THE FORMULA